Jitter producing circuitry and methods

ABSTRACT

To facilitate measurement of the jitter tolerance of circuitry such as serializer/deserializer (SERDES) circuitry, test circuitry is provided that can add jitter to a data signal. The jitter added is preferably controllable and variable with respect to such parameters as jitter frequency (i.e., how rapid is the jitter) and/or amplitude (i.e., how large or great is the amount of the jitter).

This application claims in a continuation of U.S. patent applicationSer. No. 10/846,731, filed May 13, 2004, which claims the benefit ofU.S. Provisional Application No. 60/535,907, filed Jan. 12, 2004, eachof which is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to circuitry and methods for causing a signal tojitter, e.g., to facilitate testing of the jitter-tolerance of circuitryreceiving the signal.

An example of circuitry that may need to be tested for jitter-toleranceis serializer/deserializer (SERDES) circuitry. SERDES circuitry may beused in a transmitter for converting data supplied as a succession ofparallel words to a continuous stream of serial bits. Circuitry thatreceives this serial data signal may use another SERDES to recover thesuccessive bits from the received signal and reassemble those bits intosuccessive parallel words for further processing. Clock data recovery(CDR) techniques may be used as part this data recovery operation. (Theterm “words” is used herein to mean any plural number of bits that maybe treated as a significant unit of information. For example, a word maybe eight bits; but a word can also be any other plural number of bitssuch as ten bits or 16 bits. There is no special significance to the useof the term word herein, and other terms such as nibble, byte, or groupcould have been used instead with no change in scope or coverage.)

In real-world applications the serial data signal received by a receiveris rarely, if ever, perfect. One of its imperfections may be jitter.Jitter is variation in the timing of transitions in the binary level ofthe received signal. Such transitions should occur only at boundariesbetween unit intervals (UIs) in the data signal. A UI is the timeduration of any one bit in the data signal. It is not necessary for thedata signal to transition after each UI; but when a transition doesoccur, it should be at the end of one UI and the start of the next UI.Because the UI is a fixed amount of time, transitions in the receivedserial data signal should occur only at certain times relative to oneanother (i.e., integer multiples of the UI). This fact may be used by aSERDES to help it synchronize its operations (e.g., its data recoveryoperations) to the incoming serial data signal. However, jitter cancause the timing of transitions in the received data signal to deviatefrom proper timing. For example, jitter can cause a transition in thereceived data signal to be delayed by some fraction of a UI, or to occurearlier than it should by some fraction of a UI. A SERDES should be ableto tolerate some amount of jitter without losing its ability tocorrectly recover received serial data.

Known automatic test equipment (ATE) for production testing is not welladapted to producing serial data signals with jitter to facilitateproduction testing of the jitter tolerance of SERDES or other receivercircuitry. It would therefore be desirable to provide circuitry andmethods for facilitating the use of automatic test equipment to test thejitter tolerance of circuitry such as SERDES circuitry.

SUMMARY OF THE INVENTION

In accordance with this invention, jitter can be added to a serial datasignal by adding jitter to the clock signal that is used as the timebase for the data signal. Jitter may be added to the clock signal bydelaying that signal by a time-varying amount. In the presentlypreferred embodiments, the amount of this delay varies cyclically overtime. The frequency of this cyclical variation may be controllable toallow variation of the frequency of the jitter. Alternatively or inaddition, the maximum amount of the time delay variation may becontrollable to allow variation of the magnitude or amplitude of thejitter. The data signal to which jitter has been added can be used totest the jitter-tolerance of circuitry that receives and must recoverdata from that signal. For example, circuitry for adding jitter to adata signal can be included in devices that are going to be tested(e.g., production-tested) using automatic test equipment (ATE). Such adevice can then be tested using ATE and can itself generate a datasignal having jitter for use in testing other components of the device(or other devices). Modification of the ATE is not required. Theinvention can be implemented in apparatus and/or method embodiments.

Further features of the invention, its nature and various advantages,will be more apparent from the accompanying drawings and the followingdetailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of an illustrative embodiment ofcircuitry constructed in accordance with the invention.

FIG. 2 is an illustrative graph of circuit operation information that isuseful in explaining certain aspects of the invention.

FIG. 3 is an illustrative graph of signal information that is useful inexplaining certain aspects of the invention.

FIG. 4 is a graph showing illustrative modification of the FIG. 3 signalinformation in accordance with the invention.

FIG. 5 is similar to FIG. 2, but is redrawn in relationship to FIG. 4.

DETAILED DESCRIPTION

FIG. 1 shows an illustrative embodiment in which the invention isimplemented largely as self-test circuitry that has been included in adevice under test (DUT) 10. DUT 10 may also include other conventionalcircuitry that is not shown in FIG. 1; and that other circuitry can beof many different types and/or forms. Later it will be explained thatimplementing the invention as self-test circuitry is only one of manypossibilities, and that the invention can be alternatively implementedin other ways and in other contexts if desired. For example, circuitryof the type shown in FIG. 1 (or at least the transmitter portion of thatcircuitry) can become part of test equipment (e.g., ATE) for use intesting the jitter tolerance of other devices.

In the illustrative embodiment shown in FIG. 1, DUT 10 includes fourSERDES circuits 20-A1, 20-A2, 20-B1, and 20-B2, each of which can beconventional. Each SERDES circuit 20 receives a clock signal 22-A or22-B, and may use the clock signal it receives to synchronize dataoutput or transmitter operations of the SERDES. Such transmitteroperations may include converting successive words of parallel data to aserial data output signal 24. This may include multiplying the frequencyof the received clock signal 22 within the SERDES for at least some ofthe clock requirements of the SERDES. The data that each SERDES 20outputs via its serial data output lead 24 can come from elsewhere(e.g., other circuitry on or off DUT 10), or it can be test datagenerated by the SERDES itself.

Another typical capability of each SERDES 20 is to receive a serial datasignal 26 and convert that signal to successive words of parallel data.Each SERDES 20 may output the parallel words of data that it recovers toother circuitry on or off DUT 10, or (especially in a test mode ofoperation) the SERDES may use that data internally (e.g., comparing itto expected data to test whether it is correctly recovering data fromincoming signal 26).

An overview of the remaining circuitry shown in FIG. 1 will now beprovided. A master reference clock signal (REF_CLK) is supplied on lead30. This signal can come from any suitable source on or off DUT 10. Inthe particular embodiment shown in FIG. 1 it is assumed that REF_CLKcomes from test equipment (e.g., ATE) external to DUT 10. Elements 40,50, 60, and 70 operate to produce (on lead 72) a version of REF_CLKhaving jitter. The frequency and/or amplitude of this jitter can bevaried if desired. Multiplexer 80-A allows either the signal on lead 30(REF_CLK) or the signal on lead 72 (REF_CLK with jitter) to be selectedas the reference clock signal 22-A used by SERDES 20-A1 and 20-A2.Multiplexer 80-B allows a similar selection between signals 30 and 72for the reference clock signal 22-B used by SERDES 20-B1 and 20-B2. Theselection control signals (SEL_GRP_A and SEL_GRP_B) for multiplexers 80can come from any suitable source on or off DUT 10. In the particularembodiment shown in FIG. 1 it is assumed that SEL_GRP_A and SEL_GRP_Bcome from test equipment external to DUT 10.

Continuing with the overview discussion, the FIG. 1 arrangement allowsthe reference clock signal 22-A applied to the group A SERDES (i.e.,20-A1 and 20-A2) to be the REF_CLK-with-jitter signal 72. At the sametime, the reference clock signal 22-B applied to the group B SERDES(i.e., 20-B1 and 20-B2) can be the REF_CLK signal 30 without jitter.SERDES 20-A1 and 20-A2 can then be operated to output serial test datasignals 24-A1 and 24-A2. Because SERDES 20-A1 and 20-A2 are operatingwith a reference clock signal 22-A having jitter, the serial datasignals 24-A1 and 24-A2 will have similar jitter.

Output signal 24-A1 is applied to SERDES 20-B1 input 26-B1 via one ofleads 90. (Leads 90 are shown as external to DUT-10 and are assumed inthis embodiment to be connections that are established temporarily fortesting purposes. It will be understood, however, that other ways ofproviding connections like 90 are also possible, including providingthem as selectively usable connections on DUT 10.) If SERDES 20-B1 isable to correctly interpret the jittery data signal 26-B1 it receives,it is judged to be tolerant of that amount of jitter. SERDES 20-B1 mayitself be able to determine whether it is correctly interpreting data,and may produce an output signal indicating when it is or is notachieving such correct interpretation. Alternatively, other circuitry(e.g., the test equipment testing DUT 10) may be used to receive thedata SERDES 20-B1 recovers and to determine the correctness of thatdata. As mentioned above, the FIG. 1 circuitry may allow the jitter ofsignal 72 and therefore the jitter of signal 24-A1/26-B1 to be varied infrequency and/or amplitude. The ability of SERDES 20-B1 to toleratejitter can thereby be tested over a range of jitter frequencies and/oramplitudes if desired.

At the same time that SERDES 20-B1 is being tested for tolerance tojitter in its incoming serial data signal 26-B1, SERDES 20-B2 can betested for its tolerance to jitter in the similarly jittery serial datasignal 26-B2 it receives via another one of leads 90 from the serialdata output 24-A2 of SERDES 20-A2.

After SERDES 20-B1 and 20-B2 have been tested for jitter tolerance asdescribed above, the process can be reversed to test the jittertolerance of SERDES 20-A1 and 20-A2. For example, the states ofmultiplexers 80-A and 80-B may be reversed so that SERDES 20-B1 and20-B2 receive the jittery reference clock signal (from lead 72) andSERDES 20-A1 and 20-A2 receive the no-jitter reference clock signal(from lead 30). The jittery serial data output signal 24-B1 of SERDES20-B1 is applied via one of leads 90 to the serial data input lead 26-A1of SERDES 20-A1 to test the jitter tolerance of that SERDES. Similarly,the jittery serial data output signal 24-B2 of SERDES 20-B2 is appliedvia another of leads 90 to the serial data input lead 26-A2 of SERDES20-A2 to test the jitter tolerance of that SERDES.

After all desired jitter-tolerance testing has been performed, all ofSERDES 20 can be operated with a normal REF_CLK signal from lead 30.

We turn now to a more detailed consideration of elements 40, 50, 60, and70 in FIG. 1. Clock divider circuitry 40 receives REF_CLK signal 30 anda frequency division parameter value via leads DIV[9:0]. FIG. 1 showsthe DIV[9:0] signals coming from an external source such as the testequipment being used to test DUT 10. It will be understood, however,that these signals can come from any suitable source on or off DUT 10.The value of parameter DIV[9:0] is preferably variable over time.Circuitry 40 produces a clock-type output signal 42 (divided clock orjitter control signal) having a frequency which is the REF_CLK signalfrequency divided by the current value of parameter DIV. The frequencyof divided clock signal 42 is at least partly determinative of thefrequency of the jitter given the version of the reference clock signalon lead 72. The frequency of this jitter can therefore be changed bychanging the value of the DIV parameter (assuming no change in theMAX_COUNT parameter discussed below). Increasing the value of DIVdecreases the jitter frequency, and vice versa (again assuming no changein the MAX_COUNT parameter). In the illustrative embodiment beingdescribed, DIV[9:0] can have any value from 1 to 1024. It will beunderstood, however, that this is only an example, and that any desiredrange of values can be used for this parameter.

When enabled by an up output signal from state machine circuitry 60,up/down counter circuitry 50 responds to each cycle of the signal onlead 42 by incrementing a count it maintains and outputs via leads 52(the signals DELAY_SET[6:0]). On the other hand, when state machine 60is outputting a down signal, counter circuitry 50 decrements its countin response to each signal 42 cycle.

The operations of state machine 60 are controlled in part by theMAX_COUNT[6:0] signals it receives. If the value of the parameterrepresented by the MAX_COUNT signals is 0, state machine 60 enters orremains in a “no operation” state, in which it asserts neither up nordown. Accordingly, no jitter will be produced. On the other hand, if thevalue of the MAX_COUNT parameter is not 0, state machine 60 will assertup until DELAY_SET equals MAX_COUNT. Then state machine 60 will assertdown until DELAY_SET equals 0. Then up will be asserted again, and soon, so that counter 50 repeatedly counts up and down between 0 andMAX_COUNT. It will soon become apparent how the value of parameterMAX_COUNT controls the amplitude of the jitter given to the signal onlead 72. MAX_COUNT can be varied to vary jitter amplitude if desired.(MAX_COUNT also has an effect on jitter frequency, as will be madeclearer below.) FIG. 1 shows the MAX_COUNT signals coming from the testequipment being used to test DUT 10. But it will be understood thatthese signals can come from any suitable source on or off DUT 10.

In addition to being applied to state machine 60, the DELAY_SET[6:0]output signals 52 of up/down counter 50 are applied to glitch-freecontrolled delay line circuitry 70. This circuitry can delay the REF_CLKsignal it also receives by any of many different amounts of delay, theamount of that delay being controlled by the current value of theDELAY_SET parameter. Output 72 of circuitry 70 is this selectivelydelayed REF_CLK signal.

An illustrative construction of circuitry 70 includes a plurality ofsignal delay circuit elements connected in series. For example, each ofthese delay circuit elements may delay the signal applied to it by 20pS. One hundred of these elements may be connected in series, therebyproviding a maximum delay of 2 nS. Output signal 72 may be derived fromthe output of any of these 100 delay elements, the current value ofDELAY_SET controlling that selection. Accordingly, in this exampleDELAY_SET may have any value from 0 to 100. Of course, if MAX_COUNT isless than 100, then the highest value DELAY_SET will reach will beMAX_COUNT, not 100. Also, in this example the maximum value thatMAX_COUNT can have is 100. It will be understood, however, that theseparticular values are only illustrative, and that the circuitry can beconstructed to support (1) any amount of incremental delay of REF_CLK,and (2) any number of such increments.

Circuitry constructed in accordance with the invention may be capable ofa wider range of operation, but in any particular test it will generallybe desirable to limit the amplitude of the jitter (i.e., the maximumamount of delay of REF_CLK by circuitry 70) to some fraction of UI. Thefrequency of the jitter is also logically limited to a fraction of theexpected maximum serial bit rate of the circuitry being tested.Moreover, there may be a relationship between these two variables,because most systems to be tested will probably be able to toleratehigher amplitude jitter at lower jitter frequencies, but only loweramplitude jitter at higher jitter frequencies. In any event, thecircuitry of this invention is able to provide any desired combinationof jitter frequency and amplitude.

FIGS. 2-5 are provided to ensure that the concepts of frequency andamplitude of jitter are clear. FIG. 2 is a plot of the amount by whichsignal 72 is delayed relative to signal 30 as a test proceeds withparticular values for jitter frequency and amplitude. (FIG. 2 can alsobe thought of as a plot of the DELAY_SET parameter value over time.) Thepeak-to-peak “magnitude” of the jitter is the maximum amount of delay ofsignal 72 relative to signal 30. This is computable asMAX_COUNT*TAP_DELAY, where TAP_DELAY is the delay incrementcharacteristic of circuitry 70. (Alternatively, jitter “amplitude” maybe thought of as one-half the peak-to-peak excursion shown in FIG. 2, inwhich case amplitude will be computed as MAX_COUNT*TAP_DELAY/2.) Theperiod of the jitter is the time required for the delay of signal 72relative to signal 30 to go from 0 to maximum and then back to 0 again.Jitter frequency is the reciprocal of jitter period, which is computableas FMOD=REF_CLK/(2*MAX_COUNT*DIV). It will thus be seen that jitterfrequency is a function of both DIV and MAX_COUNT.

FIG. 3 shows the UIs in a serial data signal with no jitter. (The timescale of FIG. 3 is different from that of FIG. 2, but the same as thatof FIG. 4). FIG. 3 shows the locations of all possible transitions inthe data signal, and therefore the measure of UI for the depictedsignal.

FIG. 4 shows the addition of jitter to the FIG. 3 signal information inaccordance with this invention. FIG. 4 shows that this jitter can causeeach possible transition in the FIG. 3 signal to be somewhat delayed(typically by some fraction of a UI). The maximum amount of this delayis labelled as the “magnitude” of jitter in FIG. 4.

FIG. 5 is plotted on a time axis that is perpendicular to the FIG. 4time axis (and with magnitude of delay in FIG. 4 transferred to themagnitude axis in FIG. 5) to show that over time the amount of delay inthe FIG. 4 jitter alternately increases and decreases. FIG. 5 istherefore identical to FIG. 2, but rotated 90° and linked to oneillustrative transition time in FIG. 4.

It will be understood that the foregoing is only illustrative of theprinciples of the invention, and that various modifications can be madeby those skilled in the art without departing from the scope and spiritof the invention. For example, the circuitry shown as DUT 10 in FIG. 1(or at least part of that circuitry) can be made part of test equipment(e.g., ATE) for testing the jitter tolerance of other devices. One ormore of the serial data outputs 24 of the FIG. 1 circuitry would then beconnected to the serial data inputs (similar to 26) of the SERDES orother receiver circuitry of another device to be tested for jittertolerance. That other device could also receive the REF_CLK signalwithout jitter. The FIG. 1 circuitry would be operated generally asdescribed above to produce one or more serial data output signals 24with jitter. The ability of the SERDES or other receiver circuitry inthe other device to correctly interpret that jittery data would providea measure of the jitter tolerance of the other device.

As used herein and in the appended claims, the word “successive” doesnot necessarily mean immediately following. It can just mean later intime.

1. A method of adding jitter to a clock signal comprising: delayingsuccessive cycles of the clock signal by a cyclically changing amount;and cyclically changing the amount of delay by changing the number ofunit delay increments.
 2. The method defined in claim 1 wherein thecyclically changing comprises: controlling frequency of the cyclicallychanging amount of delay.
 3. The method defined in claim 1 wherein thecyclically changing comprises: controlling amplitude of the cyclicallychanging amount of delay.
 4. The method defined in claim 2 wherein thecontrolling frequency is at least partly responsive to frequency of theclock signal.
 5. The method defined in claim 4 wherein the controllingfrequency comprises: dividing the frequency of the clock signal by adivisor parameter.
 6. The method defined in claim 5 wherein the divisorparameter is variable.
 7. The method defined in claim 2 whereincontrolling the frequency comprises: selecting a rate for cyclicallychanging the amount of delay by the plurality of unit delay increments.8. The method defined in claim 3 wherein the controlling amplitude is atleast partly responsive to an amplitude parameter.
 9. The method definedin claim 8 wherein the amplitude parameter is variable.
 10. Apparatusfor adding jitter to a clock signal comprising: means for delayingsuccessive cycles of the clock signal by a cyclically changing amount;and means for cyclically changing the amount of delay by changing thenumber of unit delay increments.
 11. The apparatus defined in claim 10wherein the means for cyclically changing comprises: means forcontrolling frequency of the cyclically changing amount of delay. 12.The apparatus defined in claim 11 wherein the means for controllingfrequency comprises: means for selecting a rate for cyclically changingthe amount of delay by the plurality of unit delay increments.
 13. Theapparatus defined in claim 10 wherein the means for cyclically changingcomprises: means for controlling amplitude of the cyclically changingamount of delay.
 14. The apparatus defined in claim 11 wherein the meansfor controlling frequency is at least partly responsive to frequency ofthe clock signal.
 15. The apparatus defined in claim 14 wherein themeans for controlling frequency comprises: means for dividing thefrequency of the clock signal by a divisor parameter.
 16. The apparatusdefined in claim 15 wherein the divisor parameter is variable.
 17. Theapparatus defined in claim 13 wherein the means for controllingamplitude is at least partly responsive to an amplitude parameter. 18.The apparatus defined in claim 17 wherein the amplitude parameter isvariable.
 19. Apparatus for adding jitter to a clock signal comprising:delay circuitry that delays successive cycles of the clock signal by acyclically changing amount; and control circuitry that cyclicallychanges the amount of delay by changing the number of unit delayincrements.
 20. The apparatus defined in claim 19 wherein the controlcircuitry comprises: frequency control circuitry that controls frequencyof the cyclically changing amount of delay.
 21. The apparatus defined inclaim 20 wherein the frequency control circuit comprises: circuitry thatselects a rate for cyclically changing the amount of delay by theplurality of unit delay increments.
 22. The apparatus defined in claim19 wherein the control circuitry comprises: amplitude control circuitrythat controls amplitude of the cyclically changing amount of delay. 23.The apparatus defined in claim 20 wherein the frequency controlcircuitry is at least partly responsive to frequency of the clocksignal.
 24. The apparatus defined in claim 23 wherein the frequencycontrol circuitry comprises: frequency dividing circuitry that dividesthe frequency of the clock signal by a divisor parameter.
 25. Theapparatus defined in claim 24 wherein the divisor parameter is variable.26. The apparatus defined in claim 22 wherein the amplitude controlcircuitry is at least partly responsive to an amplitude parameter. 27.The apparatus defined in claim 26 wherein the amplitude parameter isvariable.